High density optical recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Pat. No. 4,097,895, entitled "MULTI-LAYER OPTICAL RECORD," issued on June 27, 1978, to F. W. Spong, relates to an optical disc record/playback system wherein data are recorded on the surface of a recording medium. In a Spong system the thermal energy of a focused high intensity light beam causes variation in the optical properties on the surface of the recording medium. For example, in one system the thermal effects of a laser beam form pits in an absorptive coating on the surface of an optical disc. In the Spong system, approximately 10.sup.11 bits of information can be recorded on one side of a disc-shaped record medium having a thirty centimeter diameter.
Although the system of the previous example is directed toward archival, or permanent, storage, there have developed in recent years erasable media and recording techniques. Currently, magneto-optic materials are used in the state-of-the-art recording media to effect readily alterable recordings. A magneto-optic recording medium is a magnetic material which causes the polarization angle of laser light to be changed when reflected from a recorded spot. Preparatory to recording or playback operation, the molecules of the magneto-optic material across the entire recording surface are vertically oriented in one direction. During recording, the molecules of the magneto-optic material at the point of incidence of a laser beam are flipped in the opposite direction due to the presence of a vertical magnetic field of a given strength and polarity and the heating induced by the laser. Illumination from a playback laser beam of lower power reflected from this area will show a polarization angle change. The erasing process then restores the molecular orientation condition which existed prior to recording by the use of a vertical magnetic field of polarity opposite to that used for recording in the presence of a continuous wave laser beam.
The present invention address the problem of providing a reversible, vertically-oriented magnetic field in the region of the recording surface at the point of incidence of the laser beam. Traditionally, an electromagnet in which current (and, thus, the induced field) can be adjusted and reversed, has been the preferred source for generating a magnetic bias. Because of the very close proximity between the objective lens of the record/playback optics, however, there is very little space on the recording side of the disc for an electromagnet capable of providing a vertical magnetic field at the point of incidence, without expending large amounts of power, typically in excess of 15 watts.
There are also, however, disadvantages to backside magnetic biasing devices, those devices which apply a magnetic field from the reverse side of the disc opposite the recording/erasing beam. Because backside magnetic bias devices typically use air return paths resulting in high gradients of magnetic flux density through the disc, large amounts of power, in the order of 15-20 watts, are required to generate sufficient strength of a vertical magnetic field, typically 300 oersteds at the point of incidence of the recording/erasing beam.
The problems of a backside magnetic biasing device are compounded in the case where the disc has magneto-optic recording media on both surfaces, as where two single-sided discs are combined, resulting in a double-thickness, double-sided disc. The gradient of magnetic flux density is even greater through the double-thickness disc, and in order to provide the necessary field strength at the recording surface remote from the biasing device, the field intensity at the surface adjacent the biasing device may be sufficient to inadvertantly alter the data recorded thereon, even without the presence of laser heating.